1. Field of the Invention
The present invention relates to a high-pressure fuel pump for a cylinder injection type engine and, more particularly, to a cylinder injection high-pressure fuel pump which prevents pulsations from spreading to a low-pressure pipe.
2. Description of Related Art
A diesel engine has been widely known as an engine designed to inject fuel in the cylinders of the engine which is referred to as a cylinder injection engine or a direct injection engine. In recent years, the cylinder injection type has been proposed also for a spark ignition engine or a gasoline engine. In such a cylinder injection engine, a fuel pressure of approximately 5 MPa, for example, is necessary because the fuel is injected into a cylinder during the compression stroke of the cylinder, whereas the fuel pressure is approximately 0.3 MPa in the case of a conventional engine wherein a fuel-air mixture is produced outside a cylinder.
To obtain such a high fuel pressure, a high-pressure fuel pump is generally provided on the side of a fuel injector in addition to a low-pressure fuel pump provided in a fuel tank. In general, the low-pressure fuel pump is driven by, for example, a motor or the like and it is driven at all times as long as the power is ON, while the high-pressure fuel pump is driven by an engine and it runs as the engine runs. The high-pressure fuel pump is provided with a pulsation absorber to absorb the pulsation that takes place in the pipe at the low pressure end so as to stabilize the discharge of the high-pressure fuel pump.
FIG. 9 is a side view illustrating a conventional high-pressure fuel pump, a part thereof being shown in a sectional view; and FIG. 10 is a system diagram of the pulsation absorber on the low pressure end. In the drawings, a high-pressure fuel pump assembly 100 has a casing 1, a cylinder 30 being provided at the bottom of the casing 1; and a plunger 31 is provided in the cylinder 30 such that it is able to reciprocate therein. The cylinder 30 and the plunger 31 constitute a fuel pressurizing chamber 32.
Formed on one side surface of the casing 1 is an inlet port 14 to which a low pressure pipe (not shown) extending from the low-pressure fuel pump is connected. An inlet passage 2 is formed between the inlet port 14 and the fuel pressurizing chamber 32; a filter 8 is provided at the boundary of the inlet port 14 and the inlet passage 2. The fuel supplied from the low-pressure fuel pump is fed into the fuel pressurizing chamber 32 through the inlet passage 2. Formed also on one side surface of the casing 1 is a discharge port 34 to which a high pressure pipe (not shown) extending to a fuel injector is connected. A discharge passage 35 is formed between the discharge port 34 and the fuel pressurizing chamber 32; the fuel which has been pressurized in the fuel pressurizing chamber 32 passes through the discharge passages 35 to be discharged outside. A resonator 36 is provided in the middle of the discharge passage 35.
The plunger 31 reciprocates in the cylinder 30; it takes fuel into the fuel pressurizing chamber 32 where it pressurizes the fuel, then discharges it outside through the discharge passage 35. The high-pressure fuel pump assembly 100 is a single-cylinder type which has the single cylinder 30. Hence, oil impact occurs at every intake or discharge operation in the inlet passage 2 and the discharge passage 35, causing the fuel to pulsate. In particular, the pulsation taking place in the inlet passage 2 causes the outflow of the high-pressure fuel pump assembly 100 to drop and also causes the low pressure pipe connected to the inlet port 14 to vibrate, producing noises.
Formed on one side surface of the casing 1 is a low-pressure-end pulsation absorber 46 which has an approximately cylindrical sleeve 15 and a bottomed cylindrical piston 20 which is slidably disposed in the sleeve 15. The piston 20 is urged by a spring 23 to the right in FIG. 9. The sleeve 15 and the piston 20 constitute a capacity chamber 25. The low-pressurceend pulsation absorber 46 is provided in the middle of the inlet passage 2; the capacity chamber 25 is in communication with the inlet port 14 through an inlet passage 2a, which is one counterpart making up the inlet passage 2, and it is connected with a fuel pressurizing chamber through an inlet passage 2b, which is the other counterpart making up the inlet passage 2.
The low-pressure-end pulsation absorber 46 moves the piston 20 according to the change in fuel pressure so as to absorb the fuel pulsation produced at the high-pressure fuel pump 100. More specifically, the fuel supplied through the inlet passage 2a enters the capacity chamber 25, then moves through the inlet passage 2b toward the fuel pressurizing chamber. The fuel in the inlet passage 2b pulsates as the high-pressure fuel pump 100 takes in or discharges the fuel. At this time, the low-pressure-end pulsation absorber 46 moves the piston 20 to the left in FIG. 9 when the fuel pressure is high, while it moves the piston 20 to the right in FIG. 9 when the fuel pressure is low, thereby absorbing the pulsation of the fuel in the inlet passage 2.
The fuel pulsation generated by the high-pressure fuel pump, however, has not been completely removed even when the low-pressure-end pulsation absorber 46 is provided. The pulsation that the pulsation absorber has failed to remove reaches a low-pressure pipe (not shown) which is connected to the inlet port 14 and which extends to a fuel tank across a car body. The pulsation spread to the low-pressure pipe has been posing a problem in that it vibrates the low-pressure pipe, causing abnormal noises.